**5. Energy balance and emissions**

and hemicelluloses are polysaccharides of C6 and C5 monomers, respectively, connected by β-(1–4)-glycosidic linkages. The main lignin compounds are polymers of para-hydroxyphenyl (H lignin), guaiacyl (G lignin) and syringyl (S lignin) alcohol. Pretreatment liberates hemicelluloses first because these are hydrolyzed at a faster rate. Liberation of hemicellulose separates lignin and cellulose. β-(1–4)-glycosidic linkages are broken down by pretreatment, liberating glucose from celluloses. The various methods for pretreatment of lignocellulosic materials such as sugarcane bagasse include acid hydrolysis, alkaline hydrolysis, steam or ammonia fiber expansion, organosolv, enzymatic hydrolysis, microwave and ultrasonication, and thereof combinations between these. The most common method is the dilute acid. Ozonolysis

**Table 1** shows values for bioethanol yields reported for various systems. The key to high ethanol yield is to enable the conversion of both hexoses and pentoses into ethanol. This requires the search for new microorganisms and their metabolic engineering. A leading second-generation bioethanol plant using sugarcane bagasse is operating in Brazil by the company Raizen, a joint venture between Shell and Cosan. This highly advanced integrated facility is able to boost bioethanol production by up to 50%, in addition to the first-generation plant and without expanding cultivation land use. The use of bagasse and straws allows production even during off-season for sugarcane harvest. The progressive scalingup has allowed producing 7 million liters in its first year and planned to reach a ground-

Ethanol is used as an alternative energy source in top sugarcane-producing countries such as Brazil, India and China. World production of ethanol in 2013 was about 89 GL, with 74% of the world supply coming from Brazil and the USA [1]. The increasing biofuel production causes an increase in the biomass demand for energy purposes, which poses the challenge of the fuel versus food dilemma. The use of biomass has also raised some questions about the real benefits to decrease environmental impacts of the bioenergy systems that seek to replace fossil fuels due to the greenhouse gas emissions generated during crop cultivation and processing. To avoid unintended consequences and the translocation of issues of using biomass resources, a comprehensive analysis taking into account emissions and externalities related to energy and material consumption in the whole life cycle of sugarcane-based bioenergy

> **Total (GJ/t)**

emissions for bagasse use in power generation.

0.854 2.9 3.75 0.20 118.8

1.507 2.9 4.4 0.24 101.2

**Energy ratio (output/**

**Direct process emissions (kg CO2**

**GJ)**

**/**

**input)**

has also been used to pretreat sugarcane and agave bagasse [5].

breaking 40 million liters by 2018 [29].

76 Sugarcane - Technology and Research

systems is essential to ensure their sustainability.

**Steam production for process (GJ/t)**

**System Total electricity** 

BPST system

CEST system **production (GJ/t)**

**Table 1.** Energy ratio and direct process CO2

The current major use of sugarcane bagasse is for power supply in sugar refineries, making this facilities energy self-sufficient. Depending on the process configuration and energy requirements, some of them even export electricity to grid due to the excess bagasse available [23]. As commented in the previous section, an alternative use extensively researched nowadays is in bioethanol production [24]. In this section, the energy balance and emissions of the two alternative uses of bagasse are discussed. The indicator used to compare energy balance is the energy ratio which is defined as the energy output per unit of energy input. Energy input includes the energy originally contained in the bagasse based on its higher heating value. In the case of bagasse for power generation, the only input is the bagasse itself, in the case of the bioethanol production, the input also includes steam and electricity to run the second-generation bioethanol plant.

To perform an energy balance using sugarcane for power generation, it is necessary to know the amount of steam and electricity required for the main sugar factory process. A typical electricity demand is 28 kWh/t cane and the process steam consumption of 500 kg/cane with low efficiency factory, or about 280–340 kg/t cane for modern efficient factories [33]. The balance also depends on the pressure at which the steam is generated and fed to the turbines. Using data from [22], the energy balance of a BST and CEST system on the basis of 1 ton of bagasse is shown in **Table 2**. Current efficiencies are quite low, only 20–24% and, as expected, the CEST system performs better with higher energy ratio and lower CO2 emissions per GJ of energy delivered. These values can be improved further through reduction of steam required in the sugar factory by better energy integration as well as by replacing old equipment with more efficient one. Highly efficient cogeneration systems can achieve up to more than 80% efficiency. Improvements can lead to a significant amount of surplus bagasse becoming available for other purposes such as production of bioethanol. In such a case, approximately 50% of the bagasse is sufficient to supply the energy needs of sugar mills [33].

Given the wide availability of bagasse as an agroindustrial residue, its use for bioethanol production has been widely investigated. The energy balance for this process may be less favorable as the ethanol yields can be relatively low and may require additional energy inputs. Strategies to achieve higher efficiencies in integrated systems combine (1) higher ethanol production can be achieved by the proper pretreatment and hexoses and pentoses fermentation


**Table 2.** Reported bioethanol yields from sugarcane bagasse.

process and (2) the lignin and solid residuals are used for energy production. This can be achieved by adopting a biorefinery concept in which several process technologies are combined to convert biomass into multiple products [28]. A simplified diagram of an integrated biorefinery system is shown in **Figure 3**.

**Table 3** shows the energy ratios reported for several integrated bioethanol processes in biorefineries. It can be observed that an energy ratio of up to 0.5 can be achieved using a simultaneous saccharification and fermentation process, including strategies (1) and (2) aforementioned. Energy integration using pinch analysis is also essential to reduce process utility requirements and increase energy efficiency [35, 37].

It is important to examine the life cycle emissions as the bioethanol process uses additional inputs, including enzymes, nutrients, salts, neutralizers, and so on. An average value of 6.2 kg CO<sup>2</sup> /kg ethanol has been reported [35]. More comprehensive results of life cycle assessment environmental impacts are shown in **Table 4** for the impact categories of global warming potential (GWP-100 years), abiotic resource depletion (fossil fuels), eutrophication and acidification potentials of the integrated biorefinery system in **Figure 3**. These results show that the amount of GWP can be negative due to the savings by replacing fossil fuels by ethanol and grid electricity by the power generated from lignin and biogas.

> A flexible biorefinery that can adapt production to electricity and bioethanol can be more effective and achieve economic profitability [40]. Integrated first- and second-generation ethanol production process from sugarcane leads to better economic results, especially when advanced hydrolysis technologies and pentoses fermentation are included [32]. Novel ethanol separation and purification processes such as a combination of vacuum, atmospheric and extractive distillation systems for efficient dehydration of ethanol will also help to improve the feasibility of the bioethanol route from bagasse [41]. Other sugarcane bagasse biorefinery concepts have also been studied for production of bioethanol, methane and heat [39], as well as for chemicals, electricity and fuels with succinic acid being competitive in comparison to the petrochemical-based products [42]. Thermochemical processes via gasification and Fischer Tropsch process [23], as well as gasification for cleaner electricity production from syngas has also been reported [43]. A simultaneous economic and environmental impact assessment of biorefinery systems should be performed to enable an informed decision-making as to which

**-eq/t** 

**SO2**

**Acidification, kg** 

Sugarcane Bagasse Valorization Strategies for Bioethanol and Energy Production

**Eutrophication, kg** 

**3−/t biomass**

**PO4**

http://dx.doi.org/10.5772/intechopen.72237

79

**/t biomass**

**System Reference Energy ratio** Separate hydrolysis and fermentation [34] 0.474 Separate hydrolysis and fermentation [35] 0.419 Separate hydrolysis and fermentation [35] 0.391 Simultaneous saccharification and fermentation [36] 0.5 Simultaneous saccharification and fermentation [35] 0.438

**Table 3.** Energy ratios reported for second-generation bioethanol production.

**Global Warming Potential kg CO2**

**Table 4.** Life cycle assessment (LCA) results for bioethanol production from sugarcane bagasse [38].

Value 1586.16 −176.29 0.46 0.03

**biomass**

**Abiotic resource depletion (fossil fuels),** 

**MJ/t biomass**

Although there is no clear winner in terms of energy balance and emissions, the current market has made the use of bagasse for power generation as the focus of some companies to make profits from sales for the grid. Other leading companies, such as Raizen Energy in Brazil,

Sugarcane mills are one of the major industrial facilities in tropical and developing countries, generating income and jobs in the rural agricultural sector. These important industrial systems are evolving from single product process producing sugar to sweeten drinks and food,

consider second-generation ethanol from the bagasse as a more attractive option [45].

process technology to adopt [44].

**6. Final remarks**

**Impact category**

Comparing the two options for bagasse utilization, a study shows that the use of bagasse for power generation results in lower global warming, acidification and eutrophication potentials, whereas the bioethanol production provides resource conservation (by replacing fossil fuel) and lower human- and eco-toxicity [33]. In terms of energy balance, with the use of advanced technologies and process integration, both systems are able to achieve high efficiency level up to 50% in the bioethanol case. Up to 65% of the energy from bagasse incineration can be recovered by the biorefinery system in **Figure 3**, while only 32–33% of the energy is recovered by stand-alone bioethanol production [39]. Therefore, the use of multistage steam condensing turbines, efficient boilers, as well as the integrated first-generation + second-generation system with energy recovery from solid residues and biogas from wastewater treatment is highly recommendable to achieve high efficiency levels and environmental benefits from sugarcane bagasse and sugarcane as an energy crop.

**Figure 3.** Integrated system for bioethanol production from sugarcane bagasse.


**Table 3.** Energy ratios reported for second-generation bioethanol production.


**Table 4.** Life cycle assessment (LCA) results for bioethanol production from sugarcane bagasse [38].

A flexible biorefinery that can adapt production to electricity and bioethanol can be more effective and achieve economic profitability [40]. Integrated first- and second-generation ethanol production process from sugarcane leads to better economic results, especially when advanced hydrolysis technologies and pentoses fermentation are included [32]. Novel ethanol separation and purification processes such as a combination of vacuum, atmospheric and extractive distillation systems for efficient dehydration of ethanol will also help to improve the feasibility of the bioethanol route from bagasse [41]. Other sugarcane bagasse biorefinery concepts have also been studied for production of bioethanol, methane and heat [39], as well as for chemicals, electricity and fuels with succinic acid being competitive in comparison to the petrochemical-based products [42]. Thermochemical processes via gasification and Fischer Tropsch process [23], as well as gasification for cleaner electricity production from syngas has also been reported [43]. A simultaneous economic and environmental impact assessment of biorefinery systems should be performed to enable an informed decision-making as to which process technology to adopt [44].

Although there is no clear winner in terms of energy balance and emissions, the current market has made the use of bagasse for power generation as the focus of some companies to make profits from sales for the grid. Other leading companies, such as Raizen Energy in Brazil, consider second-generation ethanol from the bagasse as a more attractive option [45].
